Wow, thats cool that they're selling those through their site now. I have an X-Meridian in my car and an X-Prelude in my desktop. Absolutely awesome cards, both of them. I think the Meridian is easily the better sounding of the two, which is why I stuck it in my car. The Preludes are primarily meant for gaming.
As for upgrading the opamps... I guess I would have to hear it first. The ones they ship on the cards are pretty decent from my understanding, and I don't know that you would be able to hear a beneficial difference in a car environment. In a quiet room with headphones, yes.... but probably not on the highway.